Fiber delivered laser is widely used in materials processing such as drilling and cutting. Compared to traditional optical beam delivery methods, fiber delivery has several advantages including high flexibility, remote laser transportability, and enhanced safety.
Fiber delivery systems often include sensor units for monitoring the overall beam delivery integrity and the process condition of the work piece. Such sensor units are particularly important for avoiding fiber failures in high power laser applications. It is also desirable to have passive protection features to enhance the system's resistance to fiber failures.
As shown in FIG. 1, an optical fiber 10 generally consists of a fiber core 11, fiber cladding 12, and one or more outer layers 13. A light beam 15 can propagate through the fiber core because of total reflection between the fiber core 11 and the fiber cladding 12.
The maximum angle that a light beam 15 injected from air into the fiber core 11 can have to ensure total reflection in the fiber core, depicted as α in FIG. 1, is often defined as the numerical aperture (NA) of the fiber and is provided by the following expression:
      NA    =                  sin        ⁢                                  ⁢        α            =                                    n            0            2                    -                      n            1            2                                ,with n0 and n1 being the refractive index of the fiber core 11 and cladding 12 respectively. Both the core 11 and the cladding 12 are transparent at the light wavelength so that the light can be transported over a long distance with little attenuation.
The primary purpose of the outer layers 13 is to increase the mechanical integrity of the fiber. The outer layers 13 often comprise epoxy, nylon and plastic materials which may have strong absorption of the light.
Laser induced fiber failures tend to result from two sources: stray light, and cladding mode light. The former is apparent while the later one is often not realized. FIG. 2 illustrates stray light 21 and the generation of cladding mode light 22. The main portion of the injected laser beam is focused into the fiber core 11 within the acceptable NA of the fiber, and then it is coupled into fiber modes that propagate through the fiber core. Stray light 21 includes light leaking over the fiber core area, and light scattered at the fiber end surface which exceeds the acceptable NA.
The stray light 21 quickly dissipates over a fairly short distance. However, if a fiber holder or the outer layers are very close to the injection end, stray light can burn them immediately or generate thermo-stress which may gradually lead to a fiber failure.
FIG. 2 also shows the generation of cladding mode light 22, wherein a small portion of the light is launched into the cladding, and then sees the air and the fiber outer layers 13 as effective cladding layers. The cladding mode light 22 is guided through both the cladding and the core and propagates through the air-surrounded portion of fiber, or bare fiber, with little attenuation. In the portion of fiber surrounded by outer layers 13, the cladding mode light 22 experiences great attenuation because there are no total internal reflections between the cladding 12 and the outer layers 13. At the joint point between the bare fiber and the portion of fiber with outer layers 13, the cladding mode light has the strongest scattering and absorption. In one experiment, the attenuation at a single joint point was measured to be greater than 10 dB. It is evident that strong cladding light can also result in fiber failures.
Stray light and cladding mode light become stronger if the fiber is misaligned. Strong stray light and cladding mode light can also be generated because of back reflection. For instance, if the output light beam is focused at a highly reflective or scattering surface, a strong beam is coupled right back to the fiber. At the output fiber end, the back reflected beam always forms an image larger than the fiber core size so that a significant portion of the light becomes stray light and cladding mode light.
The risk of fiber failure due to stray light can be readily lowered by increasing the bare fiber length. The fiber end is fixed with a light transparent fiber holder. A thin sapphire chip is often used as the fiber holder in high power applications. Unfortunately, the bare fiber does not attenuate the cladding mode light, and the thin sapphire holder can hardly strip off the cladding mode light either. Therefore, the fiber connector is still vulnerable to cladding mode light.
To minimize the risk of fiber failures, the laser injection should allow most of the input laser power to be focused onto the fiber core, so that both stray light and cladding light can be minimized. Ideally, the fiber should always be kept aligned, and the back reflection should be carefully controlled. It is also desirable to continuously monitor the fiber delivery performance, so an active control can be engaged if a potential risk arises.
One widely used technique in fiber performance monitoring is using temperature sensors to detect local temperature changes due to heat generated by stray light and cladding mode light. This method has disadvantages of slow response and low accuracy.
Another approach is to use photodetectors, such as described in U.S. Pat. No. 4,812,641 (the '641 patent). The '641 patent describes a method of monitoring the couple efficiency by measuring input and output power with photodetectors. This method, however, has its limitations, especially for pulsed lasers. Experiments show that a small amount of cladding mode light can burn the fiber even before the couple efficiency shows a noticeable change.
Another technique is described in U.S. Pat. No. 5,319,195 (the '195 patent) in which photodetectors are used to monitor the intensity of light traveling in the cladding layer (cladding mode light) in the middle section of a fiber link. A strong sensor signal would indicate a fiber misalignment or a strong back reflection. The method described in the '195 patent is purported to be much more sensitive than the direct couple efficiency monitoring method. It is somewhat inconvenient, however, to embed sensors and electronics in the middle of a fiber link.
In light of the shortcomings of the known approaches, an alternate solution is thus required.